Hydrolyzed crosslinked maleic acid methyl vinyl ether copolymers are much in demand as stabilizing agents for pharmaceutical and cosmetic formulations as described in U.S. Pat. Nos. 5,024,779 and 5,032,391. These hydrolyzed copolymers, because of their non-toxicity and stabilizing effects, also provide for controlled release of medicaments when incorporated into pharmaceutical compositions. Notwithstanding their importance, the preparation of a uniformly clear hydrolyzed maleic acid/alkyl vinyl ether cross-linked copolymer gel involves time consuming procedures similar to ageing which requires treatment of a hydrolysis mixture up to 24 hours in order to achieve a clear gel form. In many cases, the gels obtained have a hazy appearance and gel content is subject to variations in the extent of salt formation resulting from a secondary reaction between the maleic and amine components which causes a reduction in viscosity and which affects the clarity of the product. The conventional hydrolysis processes involve introducing copolymer into an aqueous base solution containing about 0.2-5.0 weight % of an organic amine neutralizer as shown in U.S. Pat. No. 5,024,779.
Accordingly, it is an object of this invention to provide a more economical and commercially feasible process for obtaining a uniformly clear gel of a hydrolyzed, crosslinked maleic acid/alkyl vinyl ether copolymer within the period of less than 150 minutes, and in most cases less than about 90 minutes.
Additionally, it is an object of this invention to provide a hydrolyzed maleic acid/methyl vinyl ether crosslinked copolymer gel containing substantially no salt contaminant.
Still another object of this invention is to produce a maleic acid/lower alkyl vinyl ether stabilizing gel having a high Brookfield viscosity.
These and many other objects of the invention will become apparent from the following description and disclosure.